Conventional beam pumping installation for pumping fluid such as oil from underground locations utilize rods which are coupled in a continuous fashion to connect a surface pumping unit to an underground or subsurface downhole well pump for the purpose of transmitting mechanical energy from the surface equipment to the subsurface pump. The individual rods comprising the string are known as sucker rods and the plurality when coupled is referred to as a sucker rod string.
Subsurface oil well pumps are generally classified as either tubing or rod pumps. In the case of tubing pumps, the barrel is run on the tubing and the plunger is run on the rod string. Rod pumps have the advantage of being more easily removed for servicing and are less susceptible to damage in running but they offer less working area for the plunger since the maximum bore of a rod pump is necessarily less than the maximum bore of a tubing pump for the same size tubing. In either case, however, pump travel length or plunger stroke is highly important in determining output, since the plunger stroke for any given pump when multiplied by the product of stroke rate and plunger area gives the volumetric productivity.
In the prior art publication "Well Design: Drilling and Production", Craft, B. C., Holden, W. R., and Graves, E. P. Jr., Prentice-Hall Inc., 1962 it is taught that the effective plunger stroke downhole differs from the polished rod stroke; it is decreased by the effects of rod stretch resulting from fluid load and rod mass; and is increased by the effect of plunger overtravel. Since the magnitudes of these increases and decreases in stroke length are affected by the mechanical properties of the rods it is evident that the effective stroke downhole can be modified by suitable manipulation of the rod materials and characteristics, and this possibility has lead to considerable development effort in this area. In particular, it is interesting that modern data-logging and computational techniques, such as prescribed in SPE paper 588 by S. G. Gibbs presented at the Rocky Mountain Joint Regional Meeting, May 1963, of the Society of Petroleum Engineers of AIME permit the matching of sucker rod properties and the make-up of the sucker rod string to the operational parameters of a given well to achieve highly favorable pumping conditions, and hence, enhanced operational economics.
Sucker rods are currently manufactured in discrete lengths of either 25 ft. or 37.5 ft. and coupled together in the field. Couplings are expensive and introduce a mechanical weakness in the sucker rod string. References have been made to the use of wire cable without couplings. However, a cable, per se. lacks the appropriate bending rigidity to properly serve the end use.
Early sucker rods were of all-metal construction as exemplified by U.S. Pat. No. 528,168 issued Oct. 30, 1894. Thereafter initial efforts to improve sucker rod performance were concerned with use of materials and design to resist corrosion and stress failure in view of the harsh environment of the well in which the rod is worked. These efforts are illustrated in prior art patents such as: U.S. Pat. No. 3,486,557 issued in 1969 to Harrison, showing a rod comprising an inner cable surrounded by an encasement of molded plastic or fiberglass in an unspecified configuration wherein the end of the encasement has a conical recess to receive a splayed end of the cable which is held therein by metal introduced into the recess while molten and wherein the outer surface of the encasement is threaded to receive a connecting sleeve that serves to transfer load between adjacent sucker rods; U.S. Pat. No. 4,063,838 issued in 1977 to Michael showing a sucker rod having a solid steel core wrapped with resin-impregnated glass filaments in which the filaments form a stratified structure and the load transfer is via the outer surface of the wrapping in a manner similar to that described by Harrison. The information supplied to illustrate the examples in Michael show that the rod described by Michael does not fulfill the load-sharing requirements specified by the present invention. An additional patent to Carlson (U.S. Pat. No. 4,205,926) also describes a wrapped-cable rod; in this latter concept, however, the sheath material contains only helically wrapped filaments and is specifically designed to sustain compressive load in an attempt to maintain the core in a state of tension after the curing step.
It is interesting to note that as early as 1959 U.S. Pat. No. 2,874,937 to Higgins disclosed a sucker rod comprised of glass fibers held together by plastic resin. Intensive work has been undertaken in the field of fiberglass sucker rod design. Fiberglass is not seriously affected by corrosion, possesses a low specific gravity and has a high tensile strength-to-weight ratio compared to steel.
In Paper SPE6851 presented at a technical meeting of SPE of AIME, Denver in October of 1977 Watkins and Haarsma described a continuous process for producing a high-volume-fraction glass rod in which glass filaments are collimated (rendered parallel to a certain line or direction), saturated with resin, ordered into a circular configuration and cured. The current technology appropriate to the manufacture of these rods is described in U.S. Pat. No. 4,360,288 to Rutledge et al. In the current fiber glass rods all of the glass fibers are arranged in a parallel bundle and are then incorporated into a matrix. The glass filaments lie in the longitudinal direction of the rod, and the mechanical behavior of the rod is controlled by the mechanical properties of the glass fibers, which have a tensile strength in strand form of at least 200K psi and a tensile modulus of approximately 10.times.10.sup.6 psi. In the transverse direction the bundle is held together by the resin material, which typically has a tensile/compressive strength of 10K psi and a modulus of 0.5.times.10.sup.6 psi. Thus the rod has strength and stiffness properties that in the longitudinal direction are approximately 20 times as great as the properties in the transverse direction, and the overall structure possesses a high degree of anisotropy.
The paper presented data on the use of rods produced according to this process. The process has been referred to as the "pultrusion" process and the resulting rods have been referred to as "pultruded" fiberglass/resin composite rods.
Pultruded fiberglass sucker rods have a number of recognized positive attributes which include: